Generally, an electrocardiogram (ECG) is used to measure the electrical conduction system of a patient's heart. Primarily ECGs are designed to pick up electrical impulses generated by the polarization and depolarization of cardiac tissues and translate them into a waveform, which is used to measure a rate and regularity of heartbeats. Additionally, ECGs may be used to measure the size and position of the chambers in a patient's heart. Commonly ECGs are used for diagnostic and research purposes.
However, today's ECG technology and devices experience some shortcomings. In particular, current 12-lead ECG recording systems cannot be used in an MRI scanner and cannot produce readable results during an MR imaging session because when the MR scanner is performing a scan, the ECG signal relayed to the ECG recording system by existing ECG cables, is obliterated by electrical signals that are induced from the MR imaging sequence's gradient pulses (typically in the 0-30 KHz frequency band). Additionally, there is risk of the cable and/or the electrode(s) heating from the radio-frequency (RF) waves induced by the RF pulses emanating from the MR scanner. Accordingly, in the construct of the ECG cables, there is a need to utilize only materials that are non-magnetic so that the cable cannot be displaced (pulled, torqued) by the MR scanner's strong static magnetic field. Traditionally, current 12-lead ECGs can be used before and/or after an MRI scan but not during the operation of the MR system.
Alternatively, there are some ECG recording systems capable of operating in and during an MRI scan, but these ECG recording systems are currently restricted in terms of their features and capabilities. In particular, in existing ECG systems that can operate during an MRI scan there are typically only three or four electrodes feeding the ECG recording systems with electrode placements confined to a small area (electrode separation of <10 cm). ECG traces derived from a three or four electrode feed, however, are not of diagnostic grade, meeting specifications which are defined by cardiology or anesthesiology professional societies, and are not as comprehensive as 12-lead ECGs, so they cannot be used to define the location of cardiac events, should they occur in the MRI bore. Instead, three or four lead electrode ECGs are typically limited to detecting the QRS complex and thereby synchronizing the MR imaging to the cardiac cycle. Additionally, three or four lead ECG monitors offer low fidelity signals (a reduced amplitude dynamic range and/or reduced frequency content) that are not suitable for diagnostic-grade applications, such as detecting the onset of acute ischemia. In addition, it is difficult to remove the Magneto-Hydro-dynamic (MHD) voltage from 3-4 lead ECGs. MHD voltage results from the flow of blood inside the MRI's static magnetic field. The MHD voltage peaks during the cardiac cycle's S-wave to T-wave segment (“ST” segment), so it can acutely mask ischemic events, which typically are seen as elevation of the ST segment during this period. MHD voltages are commonly removed using vector-cardiogram (VCG) methods, and the use of 12-lead detection affords improved VCG separation relative to 4-lead systems.
Existing multi-lead electrodes greater than three or four used in generating ECGs cannot be laid out on the chest in the same locations as they are placed outside an MR scanner for patient safety purposes due to radio frequency (RF) energy pickup by the ECG cable's lead wires. In other words, existing lead wires for ECG cables that are capable of being utilized in an MR scanner are too short and they restrict the placement of electrodes to a very tight area to reduce the induced voltages, thereby reducing the energy pickup and thus electrode heating. The short lead wires and tight grouping are necessary for the existing MRI-conditional ECG recording systems to operate safely.
Conventional diagnostic-grade 12-lead ECGs can be used before and/or after MR imaging but not during imaging, which is a substantial shortcoming for many high risk patients (e.g., for patients suffering from Ischemic disease, anesthetized/sedated, or during trauma). Moreover, the American Heart Association (AHA) has defined ECG fidelity criteria that must be present in clinical ECG recording systems. In particular, the criteria defines the maximal allowed noise level in ECG traces (e.g., 50 microvolts), the required frequency spectrum, the minimal level of ST elevation (in mV) that must be seen during acute ischemia, and the maximal level and temporal-frequency of ECG artifacts. As a result of the AHA criteria and the existing MRI-conditional ECG recording system's technological limitations, high risk patients (e.g., patients with ischemic disease, anesthetized/sedated patients, or during trauma) are excluded from MR imaging or from MRI-guided surgery or intervention because they cannot safely monitor the heart during an MRI scan. Accordingly, in view of the above-mentioned limitations, existing ECGs are not sufficient for a cardiologist to accurately assess the physiological state of a patient while they undergo an MR examination.